i have killed at least a dozen birds from 37-47 yds with #6's its all i used for 10 years. i killed one bird that i grossly mis judged at 57yds. there was a dip in the feild would never have tried it but i thought he was about 40-45. doesnt matter though. one shot dropped and flopped. when a bird hits 45 i have never even second guessed the shot and have never had one get up. the one bird that did get up was only 18 steps and i only hit him with the edge of the pattern.
bottom line is make sure your gun and choke will pattern the #6's.